The present invention relates generally to position determination, and particularly to position determination using analog broadcast television signals.
There have long been methods of two-dimensional latitude/longitude position location systems using radio signals. In wide usage have been terrestrial systems such as Loran C and Omega, and a satellite-based system known as Transit. Another satellite-based system enjoying increased popularity is the Global Positioning System (GPS).
Initially devised in 1974, GPS is widely used for position location, navigation, survey, and time transfer. The GPS system is based on a constellation of 24 on-orbit satellites in sub-synchronous 12 hour orbits. Each satellite carries a precision clock and transmits a pseudo-noise signal, which can be precisely tracked to determine pseudo-range. By tracking 4 or more satellites, one can determine precise position in three dimensions in real time, world-wide. More details are provided in B. W. Parkinson and J. J. Spilker, Jr., Global Positioning System-Theory and Applications, Volumes I and II, AIAA, Washington, D.C. 1996.
GPS has revolutionized the technology of navigation and position location. However in some situations, GPS is less effective. Because the GPS signals are transmitted at relatively low power levels (less than 100 watts) and over great distances, the received signal strength is relatively weak (on the order of −160 dBw as received by an omni-directional antenna). Thus the signal is marginally useful or not useful at all in the presence of blockage or inside a building.
A system has been proposed using conventional analog National Television System Committee (NTSC) television signals to determine position. This proposal is found in a U.S. Patent entitled “Location Determination System And Method Using Television Broadcast Signals,” U.S. Pat. No. 5,510,801, issued Apr. 23, 1996. However, the techniques disclosed suffer from several major shortcomings. The techniques cannot use signals which are severely attenuated, such that conventional analog TV receivers cannot extract synchronization timing from the horizontal synch or chrominance burst. The techniques cannot accommodate the frequency offset and the short-term instability of the analog transmitter clocks, which would cause severe position errors because the user must sequentially sample one channel after another. The techniques cannot resolve cycle ambiguities in the chrominance carrier, especially in the presence of multipath. Further, the techniques do not enable one to use signals which have variable characteristics that do not effect the performance of an analog television receiver, but considerably affect the performance of a navigation system (for example, the variable shape and duration of the blanking pulse, the horizontal synch pulse, and the chrominance burst). Further, these techniques do not make use of a signal which allows for precise ranging to an accuracy of a few meters in the presence of multipath.